How many days are in each month of the year?

Be honest. You’ve probably had to pause mid-conversation, quietly counting on your knuckles, just to figure out whether September has 30 days or 31. No shame. We’ve all been there. It’s one of those things you feel like you should know but somehow never quite locked in.

And the weird part? The reason the months have different lengths isn’t some organised, logical system. It’s a patchwork of ancient Roman politics, astronomical approximations, and a couple of emperor-sized egos.

So let’s fix that knowledge gap, properly.

The quick answer: days in each month at a glance

Here’s the straightforward breakdown before we get into the stories and the why:

Simple enough, right? But this table raises more questions than it answers. Why does February get shortchanged? Why do July and August both have 31 days when they sit right next to each other? And honestly, who decided any of this?

Let me walk you through it.

A month-by-month breakdown (with the good stuff included)

January: 31 days

January kicks off the year with a full 31-day run, and it’s named after Janus, the two-faced Roman god who looked simultaneously backward into the past and forward into the future. Fitting start for a new year, if you think about it.

For most of Roman history, January wasn’t even the first month. The original Roman calendar only had ten months and started in March. January and February were added around 713 BC by King Numa Pompilius. January got bumped to the front of the line around 153 BC for administrative reasons related to when Roman consuls took office.

31 days. Full month. Nothing controversial here.

February: 28 days (or 29)

Ah. Here we go. February is the calendar’s awkward middle child, and its story is genuinely wild.

In the original Roman calendar, winter wasn’t assigned any months at all. The Romans essentially just ignored it. When Numa Pompilius added January and February, he worked with what was left after the other months got their days. February landed at the back of the queue and got the remainder: 28 days.

Romans also had a thing about even numbers being unlucky, so most months were given odd numbers of days. February was deliberately left at 28 as a sort of sacrificial month for rituals and purification (the name comes from the Latin februum, meaning purification).

Then came the leap year adjustment.

Every four years, February gets a 29th day. This happens to align the calendar with the actual solar year, which is approximately 365.25 days long. Without this correction, our calendar would drift roughly one day every four years relative to the seasons. Over a century, that adds up fast. You’d eventually be celebrating Christmas in summer (which, depending on where you live, might already be the case, but that’s beside the point).

One handy fact: a year is a leap year if it’s divisible by 4. But there’s a catch. Century years (1800, 1900, 2100) are not leap years unless they’re also divisible by 400. So 2000 was a leap year. 2100 won’t be.

March: 31 days

Named after Mars, the Roman god of war. This was actually the original first month of the Roman calendar, which is why September, October, November, and December literally contain the Latin words for seven (septem), eight (octo), nine (novem), and ten (decem) in their names, despite being our ninth through twelfth months. The naming never caught up with the restructuring.

March getting 31 days was partly a tribute to its martial prestige. It was a big deal month.

April: 30 days

April is one of the months that historians still debate. The name might derive from the Latin aperire, meaning “to open” (as in flowers and buds opening in spring), or it may be connected to the Etruscan version of Aphrodite. Either way, 30 days.

In personal experience, April always feels shorter than it is. Maybe it’s the erratic weather or the sudden rush of activity after winter. But the calendar insists on 30, and 30 it stays.

May: 31 days

May takes its name from Maia, a Roman goddess associated with growth and spring. In some traditions, she was identified with the Greek goddess of the same name, one of the Pleiades and mother of Hermes.

31 days, right in the heart of spring in the Northern Hemisphere. It’s often cited as one of the most pleasant months in temperate climates, for whatever that’s worth.

June: 30 days

Named after Juno, queen of the Roman gods and patron of marriage. This is supposedly why June weddings became so culturally popular. Marrying in Juno’s month was considered auspicious.

30 days. No drama.

July: 31 days

Here’s where the politics gets interesting. July was originally called Quintilis, meaning fifth month (again, the old naming system). It was renamed Julius in honour of Julius Caesar after his assassination in 44 BC, largely because Caesar had been the one to reform the calendar into something resembling what we use today. The Julian calendar introduced in 46 BC was a massive overhaul, and July got 31 days as part of that restructuring.

August: 31 days

Now this one is genuinely controversial, at least historically. Augustus Caesar, Julius Caesar’s adopted son and Rome’s first emperor, didn’t want his month to have fewer days than July. The month Sextilis was renamed Augustus in his honour in 8 BC.

But here’s the thing: at the time, August only had 30 days. That wasn’t acceptable to an emperor with a reputation to maintain. So a day was taken from February (already the shortest month) and added to August. This also disrupted the original pattern of alternating 30 and 31 day months, which is why we end up with July and August both having 31 days in a row.

Some historians dispute the full accuracy of this account (the sources are a bit patchy), but it’s the most widely cited explanation and it does match the structural irregularity we see in the calendar today.

September: 30 days

As mentioned, the name means “seventh month” in Latin. September is now our ninth month. 30 days, sitting firmly in autumn for the Northern Hemisphere and spring for the Southern.

October: 31 days

“Eighth month” by name, tenth by position. October has 31 days and is culturally loaded: harvest season, Halloween, golden leaves if you’re in the right part of the world. The etymology is simple even if the timing shift is confusing.

November: 30 days

“Ninth month,” now the eleventh. 30 days. In the Northern Hemisphere, November is when the cold really settles in. In the Southern Hemisphere, it’s the beginning of proper summer warmth.

December: 31 days

The “tenth month” that’s actually the twelfth. December closes the year with 31 days and carries the weight of every major winter holiday across cultures: Christmas, Hanukkah, Yule, Kwanzaa, New Year’s Eve.

The best tricks to remember how many days each month has

The knuckle method (the classic)

This is probably the one your grandma taught you, and it still works perfectly.

Make a fist. Starting with the knuckle of your index finger, assign January to the first knuckle (raised part), February to the valley between the first and second knuckle, March to the second knuckle, and so on.

Every knuckle = 31 days. Every valley = 30 days (except February = 28 or 29).

When you run out of knuckles on one hand, start back from the first knuckle of the same hand. July ends on a knuckle. August starts back on the first knuckle again. That’s why they both get 31 days.

This actually works. I still use it occasionally and I’ve been writing about calendars long enough to feel a little embarrassed about that.

The rhyme (the even older classic)

You’ve almost certainly heard this one:

Thirty days hath September, April, June, and November. All the rest have thirty-one, Excepting February alone, Which hath twenty-eight days clear, And twenty-nine in each leap year.

It’s old-fashioned phrasing, yes. But it works, and it’s been working since the 16th century. Sometimes the old tools are the best ones.

The “JMMJAOD” pattern

For a more visual approach, notice this pattern:

Months with 31 days: January, March, May, July, August, October, December (7 months) Months with 30 days: April, June, September, November (4 months) February: its own category entirely

Once you notice that the “big” months bookend the year and cluster mid-year, the pattern sticks more easily.

Why doesn’t everyone just use equal months?

Fair question. A few reformers over the centuries have tried exactly this.

The French Republican Calendar, introduced in 1793, divided the year into 12 months of 30 days each, with 5 or 6 extra days tacked on at the end. It actually worked administratively, but it was wildly unpopular (partly because it eliminated Sundays by switching to a 10-day week), and Napoleon abolished it in 1806.

More recently, the International Fixed Calendar (also called the Cotsworth plan) proposed 13 months of 28 days each, with one extra day at the end of the year. George Eastman, of Kodak fame, was a major advocate and actually used it internally at Eastman Kodak for decades. But it never gained mainstream traction because it required every month to start on the same day of the week, which broke compatibility with too many existing systems.

The Gregorian calendar we use today, introduced in 1582 by Pope Gregory XIII and gradually adopted worldwide, is messy and historically layered. But it’s also deeply embedded in global infrastructure, religion, agriculture, and law. Changing it now would be an organisational nightmare on a civilisational scale.

How the solar year and calendar year fit together

The Earth takes approximately 365.2422 days to orbit the Sun. Not 365. Not 365.25. Roughly 365.2422.

The Julian calendar (46 BC) assumed 365.25 days, adding a leap year every four years. Close, but not quite right. Over centuries, this tiny error accumulated into a drift of about 10 days by the 1500s.

The Gregorian calendar corrected this by adjusting the leap year rule: century years are only leap years if divisible by 400. This reduces the average calendar year to 365.2425 days, which is extremely close to the actual solar year. The remaining error amounts to about one day per 3,030 years. It’s not perfect, but it’s good enough to be functionally irrelevant for any human timescale.

For a deeper read on the history of timekeeping, the Royal Museums Greenwich has an excellent breakdown that’s worth bookmarking.

Cultural and religious calendars: not everyone uses the same system

The Gregorian calendar is the globally dominant civil calendar, but it’s far from the only one in use.

The Hebrew calendar is lunisolar, meaning it tracks both lunar months and solar years. It has 12 or 13 months per year, with a leap month added 7 times in every 19-year cycle. This is why Jewish holidays like Passover and Rosh Hashanah fall on different Gregorian dates each year, but always land in the same season.

The Islamic calendar (Hijri calendar) is purely lunar: 12 months of 29 or 30 days, totalling 354 or 355 days per year. Because it doesn’t sync with the solar year, Islamic holidays like Ramadan and Eid cycle through all seasons over a 33-year period.

The Chinese lunisolar calendar uses lunar months but adds a leap month approximately every 3 years to keep pace with the solar year. Chinese New Year falls between January 21 and February 20 in the Gregorian calendar as a result.

The Time and Date website is genuinely one of the best free tools for exploring how different calendar systems interact and when they align.

Months with the most interesting historical baggage

Some months carry more cultural weight than others, and it’s worth pausing on a few.

February has historically been associated with purification, transition, and even death in some traditions. The Roman festival of Lupercalia, celebrated in mid-February, was a fertility and purification rite. St. Valentine’s Day on February 14 has fuzzy origins, but Geoffrey Chaucer is often credited with popularising the romantic association in the 14th century.

October in the Northern Hemisphere carries a specific kind of melancholy. The days shorten visibly, the light changes quality, and there’s something about autumn that humans across cultures have always found emotionally resonant. The Celtic festival of Samhain (the root of Halloween) was observed on October 31 as a time when the boundary between the living and the dead was thought to thin.

December, aside from its holiday density, contains the winter solstice in the Northern Hemisphere, typically around December 21 or 22. This is the shortest day of the year, and it’s been marked by human cultures for tens of thousands of years. Stonehenge in England is aligned to the winter solstice sunrise, suggesting its importance predates written history by millennia.

A note on month lengths in other planets’ calendars

Just for the sake of curiosity: if you lived on Mars, a “year” would last about 687 Earth days. Mars has two tiny moons, Phobos and Deimos, but they’d make terrible calendar anchors because Phobos orbits Mars faster than Mars rotates. It rises in the west and sets in the east, completing an orbit in just 7.6 hours.

Any Martian calendar would essentially be invented from scratch. NASA and space agencies have developed notional Mars calendars for mission planning, but none are standardised. The Mars Calendar project is a fun rabbit hole if you want to explore what a 24-month Martian year might look like.

Wrapping things up

Here’s what we’ve actually covered:

• January, March, May, July, August, October, and December each have 31 days.
• April, June, September, and November each have 30 days.
• February has 28 days in a standard year and 29 days in a leap year.
• Leap years occur every 4 years, except century years that aren’t divisible by 400.
• The uneven month lengths trace back to Roman calendar reforms, imperial ego, and the imprecise math of fitting a solar year into a tidy box.

The knuckle trick and the rhyme both work. Use whichever one sticks for you.

And the next time someone asks whether April has 30 or 31 days, you won’t need to check your phone.